This invention relates to the fabrication of layered semiconductor structures, and, more particularly, to the fabrication of thin, lightweight solar cells.
Semiconductor solar cells are utilized to convert light energy to usable electrical voltages and currents. Briefly, a typical semiconductor solar cell includes an interface between n-type and p-type transparent semiconductor materials. Light shining on the interface creates hole-electron pairs in addition to those otherwise present, and the minority charge carriers migrate across the interface in opposite directions. There is no compensating flow of majority carriers, so that a net flow of electrical charge results. A useful electrical current is then obtained in an external electrical circuit by forming ohmic contacts to the materials on either side of the interface.
Semiconductor solar cells may be produced from a wide variety of semiconductor materials. Silicon cells are most widely used, but it has been found that cells fabricated from p-type and n-type gallium arsenide are particularly promising. Such solar cells have higher beginning-of-life efficiency and lower degradation with time and temperature in a space environment, as compared with silicon solar cells. Gallium arsenide solar cells are therefore particularly attractive, and have already found limited use. It is expected that gallium arsenide solar cells will find increased future application, in both space and on earth, if their cost of manufacture and weight per unit area of solar cell can be reduced.
A gallium arsenide solar cell is fabricated by depositing the appropriate semiconductor layers onto a substrate, and then adding additional components to complete the cell. More specifically, a conventional P-on-N gallium arsenide solar cell is fabricated by epitaxially depositing a layer of n-type gallium arsenide onto a single crystal gallium arsenide substrate, and depositing a layer of p-type gallium arsenide over the layer of n-type gallium arsenide. A P+ layer of gallium aluminum arsenide is deposited over the layer of p-type gallium arsenide to limit surface recombination of charge carriers. A transparent cover of glass is applied over the gallium aluminum arsenide layer to protect the active semiconductor components from physical contact and radiation damage such as encountered in a space environment. Electrical contacts to the n-type and p-type layers are applied at a convenient point.
The resulting solar cell retains the gallium arsenide substrate in contact with the remaining portions of the solar cell, but the substrate performs no essential function during operation of the solar cell and adds weight. Moreover, retention of the substrate on the solar cell requires the use of a new, separate substrate for preparing each solar cell, adding significantly to the cost of preparing the solar cell because the substrate must be grown as a single crystal and then further polished and prepared for epitaxial deposition. Thus, it would be highly desirable to be able to separate the active components of the basic solar cell from the substrate, both to reduce the weight of the solar cell and to allow the substrate to be reused in preparing subsequent solar cells.
Several approaches have been suggested for process modifications whereby the active portions of the solar cell could be separated from the substrate during fabrication. In one approach, the edges of the cell are masked and the substrate is dissolved away. Such a procedure achieves weight reduction but is difficult to control, and, of course, does not allow reuse of the substrate. In another approach, it has been suggested that an intermediate layer of gallium aluminum arsenide be epitaxially deposited between the substrate and the layer of n-type gallium arsenide. The remaining active semiconductor elements are fabricated on top of this layer, as previously described, including the top layer of p+ gallium aluminum arsenide. The active elements of the solar cell are masked, leaving the intermediate layer of gallium aluminum arsenide exposed. This layer is then dissolved away to separate the active elements of the basic solar cell from the substrate. This technique has been successfully used, but is difficult to apply to mass production of solar cells because the two layers of gallium aluminum arsenide are separated by only about 50 micrometers, so that masking of the layer in contact with the p-type gallium arsenide layer is difficult. In a variation of this technqiue, access holes have been drilled upwardly through the substrate to contact the intermediate layer of gallium aluminum arsenide. A selective reagent which attacks only gallium aluminum arsenide is then contacted to the intermediate layer through the access holes, to dissolve away the intermediate layer of gallium aluminum arsenide. This procedure is very slow, as the dissolved material and fresh selective reagent must be diffused along the long, narrow access holes. Moreover, the presence of the access holes makes the substrate essentially unusable for further fabrication operations, since it is difficult to achieve subsequent epitaxial deposition over the areas where the access holes penetrate the surface of the substrate.
There therefore exists a need for a technique where the active semiconductor elements of a basic solar cell may be effectively separated from the substrate upon which the elements are deposited during fabrication. The resulting solar cell would be thin and light in weight as a result of removal of the substrate. Preferably, the technique would not dissolve or otherwise significantly damage the substrate, so that the expensive substrate would be reused successively to manufacture further solar cells. The fabrication process would also allow retention and use of the transparent glass cover, so that the solar cell would be protected and could be manipulated readily by handling the glass cover. The present invention fulfills this need, and further provides related advantages.